1. Field of the Invention
The present invention relates to pulse position modulation communications systems, for example home networking physical layer transceivers.
2. Background Art
Local area networks use a network cable or other media to link stations on the network. Each local area network architecture uses a media access control (MAC) enabling network interface cards at each station to share access to the media.
Conventional local area network architectures use a media access controller operating according to half-duplex or full duplex Ethernet (ANSI/IEEE standard 802.3) protocol using a prescribed network medium, such as 10BaseT. Newer operating systems require that a network station be able to detect the presence of the network. In an Ethernet 10BaseT environment, the network is detected by the transmission of a link pulse by the physical layer (PHY) transceiver. The periodic link pulse on the 10BaseT media is detected by a PHY receiver, which determines the presence of another network station transmitting on the network medium based on detection of the periodic link pulses. Hence, a PHY transceiver at station A is able to detect the presence of station B, without the transmission or reception of data packets, by the reception of link pulses on the 10BaseT medium from the PHY transmitter at station B.
Chipsets have being developed that enable computers to be linked together using conventional twisted pair telephone lines instead of established local area network media such as 10BaseT. Such chipsets, implemented according to the Home Phoneline Networking Alliance (HomePNA) Specification 2.0, provide the advantage that existing telephone wiring in a home may be used to implement a home network environment. However, telephone lines are inherently noisy due to spurious noise caused by electrical devices in the home, for example dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer from turn-on transients due to on-hook and off-hook and noise pulses from the standard POTS telephones, and electrical systems such as heating and air-conditioning systems, etc.
An additional problem in implementing home networks according to the HomePNA specification 2.0 is that the HomePNA specification specifies pulse transmission times relative to a prescribed multiple of reference clock cycles. Specifically, the HomePNA specification defines a TIC time as seven (7) counts of a 60 MHz clock, resulting in a TIC time having a duration of 116.667 nanoseconds: hence, the HomePNA specification requires HomePNA pulses to be transmitted on the boundaries of the 116.667 ns TIC times.
Although the TIC time can be readily generated in a HomePNA transmitter using a divide by seven counter driven by a 60 MHz clock, implementation of the acquired TIC time becomes more difficult if a 60 MHz clock is not readily available, or if a designer prefers not to use a 60 MHz oscillator. For example, existing logic within the physical layer transceiver may utilize different clock speeds requiring a difference the oscillator, such as a 32 MHz clock. In such case, the use of both a 32 MHz based oscillator and a 60 MHz based oscillator may undesirably result in an expensive transceiver.
Attempts to utilize a 32 MHz clock for generation of the {fraction (7/60)} MHz TIC times, however, results in jitter due to the phase differences between the specified transmit clock (e.g., {fraction (7/60)} MHz) and the actual transmit clock (e.g., 32 MHz). Hence, the use of an alternative transmit clock may introduce jitter that adversely affects the required low error data rate transmission.